Green Oil Contaminant Separation
S. Karukchi and T. H. Wines, ERTC Petrochemical Conference, Vienna, October, 2004
Green oil is an oligomer formed in all C2, C3 and C4 hydrogenation reactors of ethylene plants; and other petrochemical production facilities; it is a mixture of C4 to C20 unsaturated and reactive components with about 90% aliphatic dienes and 10% of olefins plus paraffins.
In the C2 acetylene hydrogenation reactor where acetylene is hydrogenated to ethylene and ethane, the most commonly used catalyst is palladium (Pd) on Alumina (Al2O3) support. Green oil polymer is formed by side reactions of the hydrogenation reaction itself, and it cannot be totally avoided. The polymer formation starts by dimerization of acetylene with hydrogen to butadiene followed by oligomerization with successive addition of acetylene molecules to a base chain molecule adsorbed to the Pd surface.
The low molecular weight fraction of the green oil vaporizes into the gas stream; while part of the heavier fraction deposit in the pores of the catalyst, the rest is carried away with the gas as fine droplets mostly < 5 micron size with concentrations of green oil in the order of 100 ppmv to 1,000 ppmv in the gas depending on the operating temperature, age of the catalyst, CO content, H2/acetylene ratio, etc.
The gas leaving the hydrogenation reactor is cooled, and more green oil condenses into fine droplets, which deposit on the downstream heat exchangers, dehydrator beds, and on the ethylene fractionator internals. These depositing droplets are polymeric and cause fouling of the equipment; thus potentially leading to expensive unplanned shutdowns to clean up the deposited green oil.
Fuel gas used for the regeneration of the dehydrators strips out the deposited green oil on the molecular sieves; the fuel gas thus becomes contaminated with the green oil. This contaminated fuel gas then potentially causes fouling of the furnace’s low NOx burner nozzles leading to lower furnace efficiency and more frequent and costly burner tip cleaning.
Different industrially used methods for the separation of green oil from the hydrogenation reactor gaseous effluent stream were evaluated including:
- Washing of the wet gas stream from the reactor with a liquid ethylene stream in a stripping tower, • Impaction of the wet gas through a packed bed,
- Separation by a mesh pad in a knock-out drum,
- Use of a high efficiency liquid-gas coalescer with specially formulated and designed filter media - the Pall Liquid /Gas Coalescer.
Of the separation options investigated, the Pall High Efficiency Liquid/Gas Coalescer was found to be the most cost effective option that achieves a suitable and optimized degree of green oil removal from the ethylene-ethane stream.